1. Field of the Invention
The present invention relates to a thin-film device technology, and more particularly to a thin-film device technology simultaneously providing thin-film devices on a same substrate.
2. Description of the Related Art
In an active matrix liquid crystal display (AMLCD), several thin-film devices are provided on a same substrate. For example, thin-film transistors (TFTs) are adopted for a peripheral driving circuit device and/or a pixel switching device, and they are formed on the same substrate.
For the peripheral driving circuit device, the thickness of a gate insulating layer under the gate electrode of the TFT must be reduced so as to enhance driving ability, i.e. to increase the output current at a certain operation voltage.
For the pixel switching device, horizontal switch (HSW) device or electrostatic discharge (ESD) protection device, however, the thickness of the gate insulating layer of the TFT must be increased to ensure reliability of a high-current and high-voltage operation.
In addition, as a necessity of the AMLCD, a capacitor generally comprises a gate insulating layer on, for example, a poly-silicon layer, and a conductive layer (with the same material as the gate electrode) on the gate insulating layer. The sandwiched structure having the gate insulating layer between the poly-silicon layer and the conductive layer forms the capacitor. Due to the rough surface of the poly-silicon layer, reducing the thickness of the gate insulating layer may cause contact between the conductive layer and the ragged poly-silicon layer, thus result in a short circuit.
Accordingly, from the standpoint of circuit design, if the aforementioned thin film device structures are to be formed on the same substrate simultaneously or in parallel, conventional TFT technology cannot provide a manufacturing process suitable for the optimum requirements of all devices, i.e., whenever the thickness of the gate insulating layer is reduced to provide a TFT with better driving ability, the TFT may not fulfill the reliability required by a pixel switching device, and short circuits easily occur in the capacitor.